1. Technical Field
This invention relates to a vehicle travel-route guidance apparatus and associated map-data recording medium that establish a route from a departure site to a destination based on map node and link information, and that perform travel guidance in accordance with this established route. More particularly, this invention relates to a vehicle travel-route guidance apparatus and an associated map-data recording medium thereof that establish a bypass route that passes through an intersection point when such a bypass route exists in a searched destination route.
2. Related Art
A conventional travel-route guidance apparatus for vehicle use sets a route from a departure site to a destination, and performs travel guidance to the destination by highlighting the route on a road map, or by giving vocal instructions as to the direction in which the vehicle should proceed.
Such a vehicle travel-route guidance apparatus typically displays a map on a display screen by dividing several roads on a map into a plurality of nodes. The nodes are made up of an intersection point, a branch point, a union point, or the like, to thereby define the respective node intervals as a series of connected links. Information relating to the foregoing several nodes and to the several links is stored as map data in a storage medium such as a CD-ROM or the like.
Accordingly, when setting a destination route, the travel-route guidance apparatus reads map data from the storage medium, searches for a route from a departure site to the set destination, and sets an optimal route. The route setting is performed by evaluation and calculation based on node and link information, such as cost calculation according to the well-known Dijkstra algorithm, which sets the lowest-cost route as the destination route.
Traffic signals are generally installed at the intersections of major roads, and vehicles travel in accordance with these traffic signals. As shown in FIG. 7, however, many intersections include a bypass route A just before an intersection I, thus permitting a right- or left-hand turn prior to the traffic signal. FIG. 7 depicts a region where traffic flow on the right is the observed. Hereinafter, such a bypass just before an intersection will be termed an internal intersection. FIG. 8 is a structural view of nodes and links in the vicinity of this intersection I with the bypass A; Nb indicates the intersection I, and Lc indicates the internal intersection A.
Meanwhile, conventionally, when the apparatus searches for a destination route, it determines the cost of a node based on the road width of the intersection, the angle of the right-/or left-hand turn, and so on. The apparatus determines the cost of a link by link length (road length), road width, and so on. For link length, actual road length is measured, rounded off, and recorded in the link information. For this reason, in a case where link length at an internal intersection is rounded off, among the routes from node Na to node Nc, the total link length of links La and Lb, which are routes passing through the node Nb depicting the interior of the intersection, and the length of a link Lc which is the internal intersection, are substantially equal. In addition, whereas there is one right-hand turn on the route passing through the intersection node Nb, there are two right-hand turns when the route passes through the link Lc, which is the internal intersection. Because of this, the cost of right-hand turns passing through the internal intersection may become high. In such a case, the route making a right-hand turn and passing through the intersection I without passing through the internal intersection A may be selected.
However, even though there is a route that always permits a right-hand turn regardless of the traffic signal condition, this route cannot be set. Therefore, the apparatus provides guidance that is counter to the driver's intentions.